It is known that two-stroke engines can slip into a four-stroke mode. In this operating mode, a combustion occurs only every second rotation of the crankshaft. In this operating mode, increased exhaust-gas values can result for the two-stroke engine. Furthermore, faulty settings can occur in setting the amount of fuel supplied during the four-stroke operation. It is therefore desirable to recognize if a combustion is occurring in every engine cycle of the internal combustion engine.
From U.S. Pat. No. 7,325,528 it is known that the four-stroke operation has an effect on the pressure level in the crankcase. From the pressure level in the crankcase alone, however, four-stroke operation can not be reliably determined, since the pressure level is also influenced by other factors such as the rotational speed or other engine parameters.